The present invention relates generally to an optical signal amplifying circuit. More particularly, the invention is concerned with an optical signal amplifying circuit which exhibits a wide-band amplification characteristic and which can be used advantageously and profitably as an optical transmission device in optical communication systems such as an optical transmitter, optical receiver, optical repeater, branching loss compensating optical amplifier or the like.
The optical amplifier for amplifying simultaneously or en bloc optical signal components contained in a wavelength-multiplexed optical signal resulting from multiplexing of a plurality of optical signals having different wavelengths is generally implemented in such a configuration in which a Mach-Zehnder filter is disposed at an output stage in an attempt to compensate for wavelength dependency of the gain. As the conventional techniques known heretofore concerning this sort of optical signal amplifying circuit, there may be mentioned one disclosed in "IEEE Photonics Technology Letters", Vol. 3, No. 8, 1991, pp. 718-720.
For having better understanding of the present invention, technical background thereof will first be reviewed briefly. FIG. 1 of the accompanying drawings is a block diagram showing, by way of example, a basic circuit structure or configuration of a conventional optical signal amplifying circuit designed for amplifying a wavelength-multiplexed optical signal, which will be described below. In the figure, reference numeral 11 denotes generally an optical signal amplifying circuit, 1 designates an optical amplifier and 22 designates a Mach-Zehnder filter.
Referring to FIG. 1, the optical amplifier 1 destined for amplifying en bloc or collectively optical signal components having respective wavelengths is implemented by using an Er (erbium)-doped optical fiber amplifier for amplifying a wavelength-multiplexed optical signal obtained by multiplexing optical signals of different wavelengths .lambda.1 to .lambda.n. The Mach-Zehnder filter 22 connected to the output of the optical amplifier 1 serves as an optical equalizer circuit. The Mach-Zehnder filter 22 exhibits a characteristic of a gradient opposite to that of gain of the optical amplifier 1 within a relatively narrow wavelength band ranging from about 1548 to 1555 nm (nanometers) and is used for the purpose of compensating for the wavelength dependency of the gain of the optical signal amplifying circuit 11.
The conventional optical signal amplifying circuit 11 described above however suffers a problem that difficulty is encountered in implementing the optical signal amplifying circuit with high reliability because of instability of the Mach-Zehnder filter 22 employed as the optical equalizer circuit with regard to the polarization dependency and temperature characteristic.
Besides, the conventional optical signal amplifying circuit known heretofore suffers another problem in that wide-band characteristic on the order of 30 nm in the bandwidth which can ensure utilization of the wide-band characteristic of the Er-doped optical fiber amplifier to a maximum cannot be realized, because the gain compensating range is limited to a range of several nanometers around the wavelength of 1550 nm for the reason that the compensation for the wavelength dependency of the gain is limited to only a narrow wavelength range due to the intrinsic characteristic of the Mach-Zehnder filter 22.
Additionally, the conventional technique described above suffers a problem that extra-band spontaneous emission light, excitation light, monitoring light, etc., can not adequately be eliminated because of rather gentle cut-off characteristic of the Mach-Zehnder filter 22.